Decorators don't work in TypeScript after following Developer Preview guide

[jparta]

Description

Steps to Reproduce

I followed this guide: https://developer.salesforce.com/docs/platform/lwc/guide/ts.html

import { LightningElement, api } from 'lwc';

export default class Test extends LightningElement {
  @api testTesting;
}

Expected Results

Typescript accepts the decorator @api

Actual Results

In VS Code, I get the following TypeScript error:

Unable to resolve signature of property decorator when called as an expression.
  Argument of type 'ClassFieldDecoratorContext<Enhancer, any> & { name: "testTesting"; private: false; static: false; }' is not assignable to parameter of type 'string | symbol'.ts(1240)

(alias) const api: PropertyDecorator
import api

Decorator to mark public reactive properties

This prevents deploying to org.

Version

{
  "name": "salesforce-app",
  "private": true,
  "version": "1.0.0",
  "description": "Salesforce App",
  "scripts": {
    "lint": "eslint **/{aura,lwc}/**/*.js",
    "test": "npm run test:unit",
    "test:unit": "sfdx-lwc-jest",
    "test:unit:watch": "sfdx-lwc-jest --watch",
    "test:unit:debug": "sfdx-lwc-jest --debug",
    "test:unit:coverage": "sfdx-lwc-jest --coverage",
    "prettier": "prettier --write \"**/*.{cls,cmp,component,css,html,js,json,md,page,trigger,xml,yaml,yml}\"",
    "prettier:verify": "prettier --check \"**/*.{cls,cmp,component,css,html,js,json,md,page,trigger,xml,yaml,yml}\"",
    "postinstall": "husky install",
    "precommit": "lint-staged",
    "ts-build": "tsc --project force-app/main/default/lwc",
    "build-and-deploy": "npm run ts-build && sfdx project deploy start"
  },
  "devDependencies": {
    "@lwc/eslint-plugin-lwc": "^1.8.2",
    "@prettier/plugin-xml": "^3.2.2",
    "@salesforce/eslint-config-lwc": "^3.2.3",
    "@salesforce/eslint-plugin-aura": "^2.0.0",
    "@salesforce/eslint-plugin-lightning": "^1.0.0",
    "@salesforce/lightning-types": "^0.2.1",
    "@salesforce/sfdx-lwc-jest": "^3.1.0",
    "eslint": "^8.11.0",
    "eslint-plugin-import": "^2.25.4",
    "eslint-plugin-jest": "^27.6.0",
    "husky": "^8.0.3",
    "lint-staged": "^15.1.0",
    "lwc": "^8.8.0",
    "prettier": "^3.1.0",
    "prettier-plugin-apex": "^2.0.1",
    "typescript": "^5.6.3"
  },
  "lint-staged": {
    "**/*.{cls,cmp,component,css,html,js,json,md,page,trigger,xml,yaml,yml}": [
      "prettier --write"
    ],
    "**/{aura,lwc}/**/*.js": [
      "eslint"
    ]
  }
}

• LWC: 8.12.2

[nolanlawson]

Thanks for opening the issue! By any chance could you share a Git repo / Gist / TypeScript playground / etc to help repro?

[jparta]

@nolanlawson

I used the code snippet I wrote above and the TS guide to create this repro: https://github.com/jparta/LWC_decorator_repro

You'll still have to follow the TS guide to get your environment to the state that I have it in.

TS guide again: https://developer.salesforce.com/docs/platform/lwc/guide/ts.html

[jparta]

@nolanlawson
Any updates?

[wjhsf]

Argument of type 'ClassFieldDecoratorContext<Enhancer, any> & { name: "testTesting"; private: false; static: false; }' is not assignable to parameter of type 'string | symbol'.ts(1240)

This looks like you may have experimentalDecorators: true in your tsconfig.json, but it should be set to false.

[jparta]

@wjhsf

Did you check the repro I linked to?

The guide states

Open tsconfig.json and make sure the experimentalDecorators flag is unset or set to false.

From repro:
https://github.com/jparta/LWC_decorator_repro/blob/main/force-app/main/default/lwc/tsconfig.json

{
    "extends": "../../../../.sfdx/tsconfig.sfdx.json",
    "include": [
        "**/*.ts",
        "../../../../.sfdx/typings/lwc/**/*.d.ts"
    ],
    "exclude": [
        "**/__tests__/**"
    ]
}

Extended tsconfig.sfdx.json, which is unmodified after being created by VS Code command SFDX: Create Project:

{
    "compilerOptions": {
        "skipLibCheck": true,
        "target": "ESNext",
        "module": "NodeNext",
        "paths": {
            "c/decoratorRepro": [
                "<path to repro>"
            ]
        }
    }
}

[wjhsf]

Looks like you're using the VS Code extension, which is known to be outdated, so you must use experimentalDecorators: true, rather than false. See #2078 (comment) for details.

[jparta]

@wjhsf
This issue is not solved.

The TS guide says

Open tsconfig.json and make sure the experimentalDecorators flag is unset or set to false.

If I use this compilerOptions, there are no TS errors and deployment works

  "compilerOptions": {
    "experimentalDecorators": true,
  }

But upon opening the page which includes the component, I get this error, with no errors in the console:

Looks like there's a problem.

Unfortunately, there was a problem. Please try again. If the problem continues, get in touch with your administrator with the error ID shown here and any other related details.

I've updated the repro to include exposing the component, a text for when it succesfully loads, and setting experimentalDecorators: true.

I remember that I tried setting experimentalDecorators: true earlier as well, and got false hope that this would solve the issue, until reloading the app page and seeing the Looks like there's a problem. error.

My previous config (built according to a previuous version of the TS guide) had

  "compilerOptions": {
    "experimentalDecorators": false,
    "target": "ESNext",
    "module": "Preserve"
  }

Which resulted in the error

│ LightningComponentBundle │ multiUseButton │ [Line: 34, Col: 0] LWC1535: Unexpected plugin compilation error: Plugin - lwc, Hook - transform, Cause - SyntaxError:                                                               │             │
│                          │                │ /home/sfdc/tools/sfdc-lwc-compiler/11.32768.0/multiUseButton.js: LWC1104: "api" can only be used as a class decorator                                                               │             │
│                          │                │ 32 |   }                                                                                                                                                                            │             │
│                          │                │ 33 | }                                                                                                                                                                              │             │
│                          │                │ > 34 | __decorate([api], MultiUseButton.prototype, "state", void 0);                                                                                                                │             │
│                          │                │ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  

[wjhsf]

I originally closed this issue because it was already reported in #2078, but after reading through that issue again, I think it's become a bit cluttered. I'll leave this one open as it's more narrowly scoped to just the incompatibility with the VS Code extension.

Sorry for the misdirection, there's more going on here than I remembered!

---

To summarize the findings in the other issue, the LWC compiler requires that decorator syntax (@api) be preserved. The required compiler options to have TypeScript preserve decorators are "experimentalDecorators": false and "target": "ESNext". If you use "experimentalDecorators": true or any different target, then the decorators get transformed (into __decorate or __esDecorate function calls), and the LWC compiler can't handle that.

LWC only began supporting "experimentalDecorators": false in v7. Older versions require the option to be set to true. And the VS Code extension currently injects types from an older version of LWC. As a consequence, if you are using the VS Code extension, you must prefix all of your decorator usage with // @ts-expect-error or // @ts-ignore. For example:

export default class extends LightningElement {
  // @ts-expect-error The Salesforce VS Code extension doesn't like modern decorators
  @api someProp?: string;
}

This issue will be resolved when the VS Code extension can be updated to use the latest version of LWC, tracked at forcedotcom/lightning-language-server#602. It also may be resolved if we can restore support for experimental decorators (#5084 (comment)), but that might not be possible.

[wjhsf]

I've submitted changes to document this issue in the TypeScript Dev Preview guide. The changes should be live next week.

[jparta]

Thanks for the pointers. I'm following the development with interest. Solid support for TS in LWC development would a boon.